1. Technical Field
The invention relates generally to roller cone drill bits. More particularly, the invention relates to a means for diverting shale or other debris from the drill bit during operation.
2. Background Art
Roller cone drill bits typically include a main body with a threaded pin formed on the upper end of the main body for connecting to a drill string, and one or more legs depending from the lower end of the main body. FIG. 1 shows an example of a drill bit leg 2 which terminates in a xe2x80x9cshirttailxe2x80x9d portion 4. A journal 8 is cantilevered from an end face 6 of the leg 2. The journal 8 rotatably supports a roller cone 10. When the roller cone 10 is mounted on the journal 8, the end face 12 of the roller cone 8 is adjacent to the end face 6 of the leg 2. In some types of roller cone bits, a clearance 14 may be provided between the end faces 6 and 12 to prevent interference between the leg 2 and the roller cone 8 during operation of the drill bit. If provided in a particular bit, the clearance 14 is normally held to fairly close tolerance to help limit axial movement of the roller cone 10 on the journal 8. A seal 16 may be provided to retain lubricant between the mating surfaces 18 and 20 of the journal 8 and the roller cone 10, respectively, and to protect the lubricant from contaminants.
The main body of the drill bit has an internal chamber and a plurality of nozzles through which fluid received in the internal chamber is directed to the exterior of the main body. Ideally, the fluid flowing out of the nozzles would flush all debris away from the drill bit as the drill bit cuts the earth formation. However, debris tends to accumulate in the clearance between the adjacent end faces of the leg and the roller cone. Accumulation of debris in the clearance between the adjacent end faces of the leg and the roller cone is undesirable for at least two reasons. First, the relative motion between the roller cone and the leg can cause the accumulated debris to abrade the end faces of the leg and the roller cone. The end face of the leg is in a vulnerable section of the leg, i.e., the juncture between the leg and the journal. As a result, abrasion of the end face of the leg weakens the leg. Secondly, the accumulated debris can damage the seal which is intended to protect the mating surfaces of the journal and the roller cone from debris and other contaminants.
Solutions to the problem of debris accumulation in the clearance between the end faces of the leg and the roller cone are not new. U.S. Pat. No. 5,056,610 to Oliver et al, for example, discloses a solution to the debris accumulation problem which includes providing a xe2x80x9cburn plugxe2x80x9d which has a debris diverting ridge formed in its face. The burn plug is secured in an aperture in the end face of the leg and oriented such that the diverting ridge deflects debris away from the clearance between the end faces of the leg and the roller cone. The burn plug is energized to urge its ridged face against the end face of the roller cone. U.S. Pat. No. 5,358,061 to Van Nguyen discloses a different solution to the debris accumulation problem, which includes providing a hard metal scraper pad to wipe debris off the end face of the roller cone. The hard metal scraper pad is mounted across a trailing end of a circumferential groove on the end face of the leg. In another debris diverter system, a combination of a mud wiper on the end face of the leg which cooperates with a groove in the end face of the roller cone is used to control accumulation of debris in the clearance between the end faces of the leg and the roller cone.
One aspect of the invention is a roller cone drill bit which comprises a bit body that is adapted to be rotated about a longitudinal axis. The bit body has at least one leg depending therefrom. The leg terminates in a shirttail portion. The shirttail portion defines a first end face. A first circumferential groove formed on the first end face has a radius equal to the radius of a tip of the shirttail portion. A journal is cantilevered from the leg. A roller cone is rotatably mounted on the journal. The roller cone has a second end face adjacent to the first end face. A second circumferential groove is formed on the second end face. The second circumferential groove has a radius which matches the radii of the shirttail tip and the first circumferential groove.
Another aspect of the invention is a roller cone drill bit including a bit body adapted to be rotated about a longitudinal axis. The bit body has at least one leg depending from it. The leg terminates in a shirttail portion. The shirttail portion defines a first end face, this first end face having a first circumferential groove defined thereon. The first circumferential groove has a radius which is smaller than a radius of a tip of the shirttail portion. The leg also includes a journal cantilevered from it. A roller cone is rotatably mounted on the journal. The roller cone has a second end face adjacent to the first end face. In one example of this aspect of the invention, the roller cone includes a second circumferential groove on the second end face.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.